1. Field of the Invention
The present invention relates to a image signal processing circuit for inputting a image signal from an image sensor or the like, executing a signal processing to image data and outputting the processed image data to an external monitor or the like.
2. Description of the Related Art
In some cases, for example, an enlargement/reduction processing, such as digital zoom, may be executed to an image signal inputted from an image sensor. The enlargement/reduction processing can be executed to the image data in a state in which the image data is stored in SDRAM (Synchronous Dynamic Random Access Memory) in the case of a system comprising the SDRAM. However, in any system not comprising the SDRAM in terms of downsizing and cost reduction, a line memory is used so as to execute the digital zoom processing. More specifically, the line memory is used to selectively process only a line to be processed in a plurality of lines arrayed in parallel in a vertical direction.
FIG. 10 is a block diagram illustrating a schematic constitution of an image signal processing circuit for executing the conventional digital signal processing in the case of the system in which the SDRAM is not included. FIG. 11 is a timing chart for describing an operation of the circuit. In FIG. 11, a reference symbol HD denotes a horizontal synchronizing signal.
An image signal is inputted to an image signal processing unit 31 from an image sensor. The image signal processing circuit 31 generates image data (luminance signal Y, color-difference signals Cr and Db) based on the inputted image signal, and outputs the generated image data from to a line memory 32 comprising SRAM. The line memory 32 fetches the image data in a state in which all of line signals are effective (D31).
The line memory 32 changes an access timing in accordance with a preset zoom magnification. When the zoom magnification is set to ½, for example, an effective data region is reduced to half. The image data is outputted to a logic unit 33 per line while a switchover to and from an effective line and an ineffective line are switched is being executed (D32). The logic unit 33 executes an interpolation processing (zoom processing) based on a timing synchronizing with the inputted image data. The interpolated image data (D33) is outputted to an output pad 34 while the effective/ineffective switchover is being executed. The image data (D33) is outputted together with an effective/ineffective discrimination flag F. The image data (D33) is further outputted to an external monitor or the like from the output pad 34. In an external processing, the effective/ineffective discrimination flag F regards an “H” period as effective, and the image data can be thereby accurately fetched.
As a related conventional technology, No. 05-233803 of the Publication of the Unexamined Japanese Patent Applications discloses a method of reducing a noise by correcting a signal itself based on a correlation between inputted signals.
In a camera system in which the image sensor is used, a substrate is increasingly downsized and an analog circuit is increasingly formed into an on-chip structure. In response to the foregoing trend, there is a concern that an image quality may be deteriorated under the influence of the noise with respect to the analog circuit. FIG. 12 shows a constitution of LS140 in which an image signal processing circuit 30 and an analog circuit 35 shown in FIG. 10 are formed into a one-chip structure.
In the foregoing conventional method, the digital processing is executed to the image data changing per clock in the signal-effective line, while there is no power consumption in the signal-ineffective line because the data does not change. Therefore, a power consumption P is largely different between a signal-effective period and a signal-ineffective period. The large variation of the power consumption P in the image signal processing circuit 30 results in the noise, which is transmitted to the analog circuit 35 via a power-supply line and a GND line. Because the analog circuit 35 is easily affected by the noise, a density of the resulting image is different in each line as shown in FIG. 13 in the case of generating the large variation in the power consumption P in the image signal processing circuit 30.
For reference, in No. 05-238803 of the Publication of the Unexamined Japanese Patent Applications, a sufficient noise reduction cannot be expected because the signal causing the noise cannot be controlled.